What's the correct format for MAKEFLAGS when using Fish shell?

Question

To improve compile times, the Arch wiki states,

Users with multi-core/multi-processor systems can specify the number of jobs to run simultaneously. This can be accomplished with the use of nproc to determine the number of available processors, e.g. MAKEFLAGS="-j$(nproc)".

If I set this in Fish shell via set -Ux MAKEFLAGS "-J$(nproc)", then I receive the error:

fish: $(...) is not supported. In fish, please use '(nproc)'.
set -Ux MAKEFLAGS "-J$(nproc)"
                     ^

I can set this variable in two ways without receiving an error:

1. set -Ux MAKEFLAGS "-J(nproc)"

2. set -Ux MAKEFLAGS '-J$(nproc)'

Which of these is the correct method? Or are they both okay?

Thanks

Answer 1

Neither. In fish, command substitution cannot be quoted.

set arg "-J(nproc)"
set -S arg

$arg: set in global scope, unexported, with 1 elements
$arg[1]: |-J(nproc)|

Use

set -Ux MAKEFLAGS "-J"(nproc)

Answer 2

As of fish 3.4.0, command substitutions can be quoted using the "$()" syntax, so the original command works now.